The present invention relates generally to laser apparatuses, and more particularly to a laser apparatus in an exposure apparatus that illuminates a mask or reticle (these terms are used interchangeably in this application) which forms a pattern for use with a lithography process for fabricating semiconductor elements, liquid crystal display devices (LCD), image pick-up devices (such as CCDs, etc.), thin-film magnetic heads, and the like.
Along with recent demands on smaller and lower profile electronic devices, fine semiconductor devices to be mounted onto these electronic devices have been increasingly in demand. As a transfer (lithography) method for fabricating semiconductor devices, a projection exposure apparatus has been used conventionally.
A critical dimension (or resolution) transferable by a projection exposure apparatus is in proportion to the wavelength of light to be used for exposure. Therefore, in recent years, the exposure light source is in transition from the conventional ultra-high pressure mercury lamp (including g-line (with a wavelength of about 436 nm) and i-line (with a wavelength of about 365 nm)) to a KrF excimer laser with a shorter wavelength (i.e., a wavelength of about 248 nm) to the ArF excimer laser (with a wavelength of about 193 nm), and practical use of the F2 laser (with a wavelength of about 157 nm) is also being promoted.
Optical elements that efficiently transmit light in such a wavelength range (i.e., in the ultraviolet region) are limited to certain glass materials such as synthetic quartz, calcium fluoride, etc., and thus it is difficult to correct chromatic aberration. Therefore, in using the KrF excimer laser and the ArF excimer laser for an exposure light source, a wavelength spectral bandwidth of about 300 pm at full width at half maximum in a spontaneous oscillation state is generally turned to a narrowband of, e.g., about 0.5 pm, and feedback control (or wavelength control) is provided by a wavelength selection element in a resonator such that a laser beam may be always oscillated with a desired wavelength while the wavelength is monitored.
On the other hand, in using the F2 laser as an exposure light source, it is impossible to turn a wavelength spectral bandwidth into a narrowband or to provide the wavelength control for technical reasons: including multiple oscillation spectra existing in the neighborhood of 157 nm; its wavelength spectral bandwidth in a spontaneous oscillation state as much as about 1 pm, i.e., narrower than the KrF excimer laser and the ArF excimer laser; difficult turning of the laser beam into a narrower band since the performance of an optical element used for a wavelength bandwidth of 157 nm has not been satisfactory enough to be put into practical use, difficult measurement of a wavelength and a wavelength spectral bandwidth with high precision inside a laser apparatus, and so on. Accordingly, the line selection method has been proposed which selects, from among several wavelengths oscillated in the F2 laser, only one wavelength for oscillation.
The F2 and other excimer laser generally require a laser apparatus that encloses halogen gases such as fluorine, etc., and rare gases such as helium, neon, etc. in a chamber, and uses electric discharges produced by applying high voltage between the electrodes disposed in the chamber to excite gases, thus oscillating the laser beam. The continuous oscillation of the laser beam would lower the concentration of the halogen gas because the halogen gas would react on impurities present in the chamber, or be absorbed by the inner wall of the chamber. Therefore, a compositional ratio of the laser gas varies from its optimum ratio, thereby causing pulse energy (or laser oscillation efficiency) to be lowered.
Accordingly, the pulse energy of the laser beam is kept at a desired value by raising the voltage to be applied between the electrodes, or by insufflating a specified amount of gas including halogen gas (gas injection) to raise the gas pressure in the chamber when a rising amount of the applied voltage reaches a certain threshold. However, the repetitive gas injection would increase impurities in the chamber, and facilitate interaction between the impurities and halogen. As a result, the pulse energy cannot be maintained at a desired value even with the increased voltage to be applied between the electrodes rises and gas injection, because. When the gas injection becomes less effective, the majority of the gas in the chamber is exhausted and fresh gas is injected (gas exchange). In other words, the F2 and other excimer laser obtain a desired output by changing the gas pressure and/or the partial pressure of fluorine in the chamber or by raising the voltage applied between the electrodes, depending on the use circumstances.
In case of the F2 laser, it has become evident that as gas characteristics such as its pressure and temperature in a chamber vary because of gas injection, etc., the oscillation wavelength and wavelength spectral bandwidth of the laser beam will change accordingly.
Accordingly, when the F2 laser is used as an exposure light source, its oscillation wavelength and wavelength spectral bandwidth will change during exposure. If they exceed, e.g., wavelength stability and a tolerance of the wavelength spectral bandwidth required by an exposure system, it becomes difficult to achieve desired resolution required for the exposure apparatus.
Moreover, when the F2 laser is used, it is difficult to directly confirm whether or not the laser beam exhibits its desired performance during exposure, because the measurement of a wavelength is technically very difficult in a laser apparatus with high accuracy. The direct confirmation of the oscillation wavelength, if any, would clearly give an adverse impact onto the productivity of the exposure apparatus in running cost and maintenance frequency in light of the current durability of a current optical element. A similar problem arises when a laser that oscillates in a wavelength shorter than the F2 laser is used as a light source.